Correct me if Im wrong but there are houses that are powered by solar panels on the roof, and in Germany I think there is an office building that generates power for the local area because it was designed with more solar panels than it needs most of the time. And photovoltaic paint/ink already exists which means it should be possilbe to have a lot more versatility in how photovoltaic energy can be gathered, for example all lampposts could be converting light to electricity during the day and do the reverse at night.

So imo mass producing solar panels and complementing this decentralized default base with some local/regional wind power and geothermal should do the trick.

A very important aspect of the energy debate is the understanding of base load requirements. Wind power and solar power are excellent sources of energy to compliment a system of other sources. Wind and Solar alone simply cannot provide a working reliable energy grid. Wind power is often touted in the media (at least where I come from - Ireland) as being the solution to all of our problems and then quasi-scientific half truths often accompany this statement by saying how much energy wind contributes to the grid. The chosen figure is often a peak output on a windy day, completely irrelevant in terms of a working grid.

As a general rule of thumb, wind energy on average only ever produces at most 20% of its rated capacity. For example a 1MW turbine can only ever be expected to produce an average of 200KW of power baseload and even that is an optimistic figure. By interconnecting wind farms one can increased this number to about 30% which is a big improvement but still a huge different from the quoted output.

Optimal turbine operation only occurs in very specific wind velocities. I cannot remember exactly but the power output falls off either as a square or cube of wind speed. What I do know is unless the wind is blowing in the relatively narrow wind speed band that the turbine is designed for, your power output is significantly impacted.

Solar power's problems are very well known and its biggest problems are obvious. How do we store the energy at night? Yes, you can pump it up hill or melt salt with it, but this drives up the cost hugely. Also, the environmental impact of solar and wind aren't fully understand. Speaking solely from a physics point of view here, taking gigawatts of solar and wind energy out of the environment is bound to have localized effects on the climate of that area (different wind patterns, different weather due to lack of solar heating..etc)

While the idea of geothermal is often touted as a sustainable solution to the base load problem, it comes with numerous problems and challenges. First and foremost it has to be noted that geothermal is not as sustainable as you might think. You have to be very, very careful to avoid localized heat depletion (there are several examples of this happening in the past). It is very bad news if all of your expensive infrastructure end up sitting on a cold spot!

Of course you can now say the inevitable ''We just need one or two breakthroughs'' or with a little bit of research and development they will be the perfect solution. Why can't nuclear fission be researched further and made safer? It is unfair to assume that nuclear fission energy research is a closed book.

The important thing to take away from this is that solar and wind are not silver bullet solutions despite what the populace believe. Nor is nuclear. The true solution to the energy crisis will be a combination of these energies. Nuclear fission is fundamental to this solution, without it there is no hope of sustainable base load.

It will be several decades until an economic fusion power plant will be online. A combination of solar, wind and nuclear need to plug the gap until this happens.

A very important aspect of the energy debate is the understanding of base load requirements. Wind power and solar power are excellent sources of energy to compliment a system of other sources. Wind and Solar alone simply cannot provide a working reliable energy grid. Wind power is often touted in the media (at least where I come from - Ireland) as being the solution to all of our problems and then quasi-scientific half truths often accompany this statement by saying how much energy wind contributes to the grid. The chosen figure is often a peak output on a windy day, completely irrelevant in terms of a working grid.

They have to be combined with a storage medium of some kind, or assigned to a purpose that always coincides with peak output (for example: using a solar panel to power your home's air conditioning unit.)

Hydrogen is the easiest choice. Electricity can be used to separate hydrogen out of water, and then the hydrogen can be used to generate electricity later. In the process you lose about 3/4 of the energy, so...... just build 4x as many solar panels as you were originally planning and there you go.

Optimal turbine operation only occurs in very specific wind velocities. I cannot remember exactly but the power output falls off either as a square or cube of wind speed. What I do know is unless the wind is blowing in the relatively narrow wind speed band that the turbine is designed for, your power output is significantly impacted.

There's no reason you couldn't put a gearbox on the turbine, just like how automobiles have gearboxes to allow the car's engine to operate near its optimal output speeds, which are also somewhat specific.

Solar power's problems are very well known and its biggest problems are obvious. How do we store the energy at night? Yes, you can pump it up hill or melt salt with it, but this drives up the cost hugely. Also, the environmental impact of solar and wind aren't fully understand. Speaking solely from a physics point of view here, taking gigawatts of solar and wind energy out of the environment is bound to have localized effects on the climate of that area (different wind patterns, different weather due to lack of solar heating..etc)

I'm glad you brought that up. I really haven't seen very many people mention this, but I worry about it a little bit too.

While the idea of geothermal is often touted as a sustainable solution to the base load problem, it comes with numerous problems and challenges. First and foremost it has to be noted that geothermal is not as sustainable as you might think. You have to be very, very careful to avoid localized heat depletion (there are several examples of this happening in the past). It is very bad news if all of your expensive infrastructure end up sitting on a cold spot!

I'd be more worried about inadvertently triggering earthquakes, for the same reasons as you mentioned for solar and wind. Sapping all that thermal energy out of the Earth's crust is bound to cause things to shift around.

Of course you can now say the inevitable ''We just need one or two breakthroughs'' or with a little bit of research and development they will be the perfect solution. Why can't nuclear fission be researched further and made safer? It is unfair to assume that nuclear fission energy research is a closed book.

Nuclear will never be popular so long as scare stories like Fukushima keep happening, and they will keep happening. People are just too greedy to take all the necessary precautions. They cut corners.

Some clocks are only right twice a day, but they are still right when they are right.

There is no real need to argue the fine details of the potential solutions to our energy needs. I think that we are in broad agreement that the only viable and likely solution will be a full spectrum of solutions from each of the various technologies.

Personally, I don't feel that a broad spectrum solution that does not include some form of nuclear base load is credible.

I'd be more worried about inadvertently triggering earthquakes, for the same reasons as you mentioned for solar and wind. Sapping all that thermal energy out of the Earth's crust is bound to cause things to shift around.

Your worries are probably very real. I do not know what the situation is like in other countries but I know that here in Ireland information like the expected impacts of geothermal are not available and they are not going to be readily available without significant time and investment. I have many ties in the Earth Science and mining industry over here and the situation is rather bleak as regards hard geology (as opposed to the much hyped ocean and marine related fields which are currently being pumped full of cash under buzzword umbrellas of green energy and climate change research).

The net result is that much of the human capital from the hard geological sciences are simply not there anymore, most are now retiring and NOT being replenished. The science as a whole is being left wither as there is no popular consensus that it is useful (a complete and total fallacy!). What are the implications of this? Well if there is to be any push on geothermal I fear that at least here in Ireland, we likely won't have the knowledge base or skills to competently carry out the research.

It's unfortunate that this story is not unique to the geological sciences, there appears to be much broader trends throughout the sciences of the ''less sexy'' sciences suffering terribly due to lack of public knowledge and understanding. Many administrative efforts look for easy to understand results and progress, something that is not always compatible with these types of sciences.

Nuclear will never be popular so long as scare stories like Fukushima keep happening, and they will keep happening. People are just too greedy to take all the necessary precautions. They cut corners.

I disagree with your assertion that corners were cut on the Fukushima reactor. It is very, very important to remember that the vast, vast majority of our nuclear reactors are archaic to say the least. These things were built in the 50's/60's/70's. There have been 50 years or scientific and engineering advancements since then so it is very unfair to criticize them.

Personal I think they are engineering marvels that have been quietly humming over the decades always providing a relatively safe and guaranteed backbone power supply. Obviously there have been accidents but when compared to all other industries the nuclear industry is strikingly well run and safe.

If you get time, I suggest you take a quick look at this website (it's an Irish one but the information is very, very good). I have linked you to the page regarding nuclear safety which is very compelling.

There is no real need to argue the fine details of the potential solutions to our energy needs. I think that we are in broad agreement that the only viable and likely solution will be a full spectrum of solutions from each of the various technologies.

Personally, I don't feel that a broad spectrum solution that does not include some form of nuclear base load is credible.

You can tell I'm more of a Hydrogen proponent. If we were able to amass large enough Hydrogen reserves, and supported solar and wind with Hydrogen powered generator plants, the problem would be solved that way as well.

Nuclear will never be popular so long as scare stories like Fukushima keep happening, and they will keep happening. People are just too greedy to take all the necessary precautions. They cut corners.

I disagree with your assertion that corners were cut on the Fukushima reactor. It is very, very important to remember that the vast, vast majority of our nuclear reactors are archaic to say the least. These things were built in the 50's/60's/70's. There have been 50 years or scientific and engineering advancements since then so it is very unfair to criticize them.

Personal I think they are engineering marvels that have been quietly humming over the decades always providing a relatively safe and guaranteed backbone power supply. Obviously there have been accidents but when compared to all other industries the nuclear industry is strikingly well run and safe.

If you get time, I suggest you take a quick look at this website (it's an Irish one but the information is very, very good). I have linked you to the page regarding nuclear safety which is very compelling.

Now that I've been reading more on it, I think saying "corners were cut" is inaccurate as well. The problem is there were just too many reactors of too great a size bunched too closely together. Nobody can hope to anticipate all possible geological events, like a Tsunami over 25 feet high.

Probably the best way to manage nuclear catastrophe dangers is to decentralize the plants into smaller plants, so a melt down never exceeds the logistical abilities of the surrounding areas. At Oregon State they've been researching "micro-nukes" where the fuel rod is about the size of a tic tac, and the plan is to build a large number of these kinds of plants with lots of concrete around them so the worst case scenario isn't really all that bad. I think the fuel-to-energy efficiency ratio suffers, but the overall safety would greatly improve.

Some clocks are only right twice a day, but they are still right when they are right.

Now that I've been reading more on it, I think saying "corners were cut" is inaccurate as well. The problem is there were just too many reactors of too great a size bunched too closely together. Nobody can hope to anticipate all possible geological events, like a Tsunami over 25 feet high.

Absolutely. Although you would be amazed at how stringent the regulations are regarding nuclear power. Most of it is driven entirely by fear and a poorly educated public.

An excellent example of this albeit slightly off topic is the Yucca mountain repository. It's a favorite criticism of the costs associated with nuclear power and the downright insane safety being asked for. The biggest reason behind the astronomical cost of Yucca and the prolonged building process was due to people (most likely Congress) continually asking ''Great you've made X,Y,Z safety features... but can you make it safer yet?''. Naturally the engineers will respond ''Well... yes, I guess we can but it's not necessary and will cost more''. This then goes in a never ending circle of safety improvements which causes skyrocketing costs and huge build times.

This can be used as a rough analogy to the long build times with nuclear power stations (a commonly cited drawback). The regulations they are forced to abide by are in some cases excessive and driven by an unfounded public fear of *shush don't say it* ***RADIATION***.

Probably the best way to manage nuclear catastrophe dangers is to decentralize the plants into smaller plants, so a melt down never exceeds the logistical abilities of the surrounding areas. At Oregon State they've been researching "micro-nukes" where the fuel rod is about the size of a tic tac, and the plan is to build a large number of these kinds of plants with lots of concrete around them so the worst case scenario isn't really all that bad. I think the fuel-to-energy efficiency ratio suffers, but the overall safety would greatly improve.

There are many new technologies being explored currently and it's unfortunate that there is not a much wider discussion on the topic. I fear that many people see nuclear fission research as being a closed book. This assertion couldn't be further from the truth sadly. There is so much that can be done with the technology to improve safety and proliferation concerns.

Both coal and hydroelectricity have lousy safety records - many times worse than nuclear.
Safety is measured here as accidental deaths per terrawatt year of electricity produced. The numbers come out as :
Coal : 342 (This includes coal mining accidents, but not deaths from respiratory disease after breathing coal smoke)Natural gas : 85 (a lot of these from gas pipeline accidents)Hydroelectricity : 883 (mainly from dam bursts)nuclear : 8 (mostly from Chernobyl)

Coal and gas are not good in terms of carbon emissions. Wind has been well critiqued already on this thread. Overall, nuclear remains one of the best options, only let down by rather irrational politics.

Last edited by skeptic; October 4th, 2011 at 07:59 PM.
Reason: add detail

Geothermal = earthquakes, but worth a try. In 2004 54% of Iceland's primary-power was in Geothermal, and combined with hydroelectricity soon Iceland will be 100% fossil fuel free.

Iceland is located directly on the crack of the earth's crust, molten lava are nearer to the surface thus Geothermal borehole is less costly in Iceland, but Geothermal can work everywhere provided that its borehole is deep enough to reach the mantel's heat: eg, the deepest hole was dug in Russia (Kola Superdeep Borehole), and they couldn't dig anymore because the drill melted (but this prove that Geothermal works, you don't need to drill that deep; just enough to boil water).

Solar power's problems are very well known and its biggest problems are obvious. How do we store the energy at night? Yes, you can pump it up hill or melt salt with it, but this drives up the cost hugely.

It's still cheaper than nukes or coal - if you keep honest books.

Overnight storage is not the problem - most thermal solar setups have that built in. It's the two or three week supply that needs greater investment in infrastructure.

Poorer, if Japan is any example. Economy is already stressed because of poor decision to shut down nuclear power installations. Nuclear power has been "stored" for millions of years, only waiting to be tapped as required. Obviously most reliable alternative which is important for industry- "Sorry, steel mill closed due to rainy day." Manufacturing is not American baseball!

American baseball is popular in Japan but new Prime Minister Yoshihiko Noda has affirmed repeatedly necessity of resuming nuclear power production in order to prevent damage to overall economy.

Last edited by The Finger Prince; October 5th, 2011 at 01:03 AM.

The bravest are surely those who have the clearest vision of what is before them, glory and danger alike, and yet notwithstanding go out to meet it.- Thucydides

There is no real need to argue the fine details of the potential solutions to our energy needs. I think that we are in broad agreement that the only viable and likely solution will be a full spectrum of solutions from each of the various technologies.

Personally, I don't feel that a broad spectrum solution that does not include some form of nuclear base load is credible.

You can tell I'm more of a Hydrogen proponent. If we were able to amass large enough Hydrogen reserves, and supported solar and wind with Hydrogen powered generator plants, the problem would be solved that way as well.

Nuclear will never be popular so long as scare stories like Fukushima keep happening, and they will keep happening. People are just too greedy to take all the necessary precautions. They cut corners.

I disagree with your assertion that corners were cut on the Fukushima reactor. It is very, very important to remember that the vast, vast majority of our nuclear reactors are archaic to say the least. These things were built in the 50's/60's/70's. There have been 50 years or scientific and engineering advancements since then so it is very unfair to criticize them.

Personal I think they are engineering marvels that have been quietly humming over the decades always providing a relatively safe and guaranteed backbone power supply. Obviously there have been accidents but when compared to all other industries the nuclear industry is strikingly well run and safe.

If you get time, I suggest you take a quick look at this website (it's an Irish one but the information is very, very good). I have linked you to the page regarding nuclear safety which is very compelling.

Now that I've been reading more on it, I think saying "corners were cut" is inaccurate as well. The problem is there were just too many reactors of too great a size bunched too closely together. Nobody can hope to anticipate all possible geological events, like a Tsunami over 25 feet high.

Probably the best way to manage nuclear catastrophe dangers is to decentralize the plants into smaller plants, so a melt down never exceeds the logistical abilities of the surrounding areas. At Oregon State they've been researching "micro-nukes" where the fuel rod is about the size of a tic tac, and the plan is to build a large number of these kinds of plants with lots of concrete around them so the worst case scenario isn't really all that bad. I think the fuel-to-energy efficiency ratio suffers, but the overall safety would greatly improve.

Where will come from this hydrogen? Hydrogen wells perhaps? And your "micro-nukes" sounds like pebble bed reactors, not a bad idea, but LFTRs are better. Looking at website provided it is easy to see that nuclear power provides abundant electricity for hydrogen production- less obvious is that TEMPERATURE ALONE can be used to sunder bonds of hydrogen and oxygen before a watt is even generated.

"Some prototype Generation IV reactors operate at 850 to 1000 degrees Celsius, considerably hotter than existing commercial nuclear power plants. General Atomics predicts that hydrogen produced in a High Temperature Gas Cooled Reactor (HTGR) would cost $1.53/kg. In 2003, steam reforming of natural gas yielded hydrogen at $1.40/kg. At 2005 gas prices, hydrogen cost $2.70/kg.[citation needed]Hence, just within the United States, a savings of tens of billions of dollars per year is possible with a nuclear-powered supply. Much of this savings would translate into reduced oil and natural gas imports." - from Wikipedia

Poorer, if Japan is any example. Economy is already stressed because of poor decision to shut down nuclear power installations. Nuclear power has been "stored" for millions of years, only waiting to be tapped as required. Obviously most reliable alternative which is important for industry- "Sorry, steel mill closed due to rainy day." Manufacturing is not American baseball!

American baseball is popular in Japan but new Prime Minister Yoshihiko Noda has affirmed repeatedly necessity of resuming nuclear power production in order to prevent damage to overall economy.

That's how farming is. You end up depending on the weather. So life has some chance associated with it. Personally I think a policy of charging less for energy on sunny days in order to encourage industrial processes such as iron working to utilize it would be a good direction to go. Either that, or making hydrogen on sunny days, then using the hydrogen later to make Methane for long term storage. (Since the two processes that are used to make artificial CH4 are both Hydrogen+ something else reactions.)

Converting to CH4 and back is ~60% efficient both ways (both making it from electricity, and converting it back to electricity), so ~36% efficient overall, but if we're talking about iron working, where the majority of the energy consumed is heat anyway, then in theory you could pipeline CH4 into the plant directly and burn it on site to make the heat, instead of using up electricity for that purpose. At that point, the overall efficiency is 60%, because converting CH4 directly to heat is almost a 100% efficient conversion.

Some clocks are only right twice a day, but they are still right when they are right.

Maybe they should work on Tidal Power. They've certainly got plenty of coast line. Probably they wouldn't even have to worry too much about the transmission costs of getting the power inland. No part of the country is more than 150 miles from the ocean.

Or if they want storage, they should dig pumped storage wells near the coast. Just use them in reverse, harness energy on cloudy days by letting ocean water flow into them, and then use energy on sunny days to pump the water back out so they can start the process over again.

Some clocks are only right twice a day, but they are still right when they are right.

Very difficult for Wiki entries to be dishonest. These entries are subject to constant scrutiny and editing. These figures have remained untouched for some time, which would suggest they are valid.

To kojax

Tidal power is probably a non starter at this point in time.

As I have said, many times, coal, gas, hydro, and nuclear make up about 97.5% of all electricity generated world wide. This is not arbitrary. The people planning power plants are hard nosed business men. They do not use much tidal power, or geothermal, or wind, because under most situations, those are not practical alternatives.

The International Atomic Energy Agency (IAEA)
released a report on Japanese efforts to control and clean up
from the Fukushima accident. Overall the 12-man team gave
high marks to the Japanese authorities for coordinating local and
central government in planning decontamination measures,
monitoring radiation levels in a wide range of areas, and
releasing detailed information. But, they urged Japan to take a
more focused and realistic approach to dealing with radioactive
contamination in areas around the stricken Fukushima Daiichi
nuclear plant in northeastern Japan, amid signs the Japanese
government is becoming overwhelmed by public demands for
decontamination, according to a Wall Street Journal report.

"They are encouraged to avoid over-conservatism which could
not effectively contribute to the reduction of exposure doses,"
said the experts in their report.

The Japanese Environment Ministry is planning to
decontaminate more than 960 square miles, or 2,400 square
kilometers, to achieve an annual radiation exposure limit of 5
millisieverts (mSv). The government estimates this would cost
more than Y1 trillion, or about $13 billion. The majority of the
area is sparsely populated woodland. There is great deal of
pressure from certain politicians, media and environmentalist
"experts" to bring down the annual exposure limit even further,
to 1 mSv. Decontamination to this level would increase the burden
of the task more than five times, since more soil from a larger
area would have to be removed, transported and stored (in
as-of-yet unallocated storage areas). (For comparison - an
American receives on average a background dose of 6.2 mSv, and
there are populated areas in Europe and India, for example, where
the background dose is over 50 mSv. One medical CAT scan can
result in a dosage of as much as 18 mSv.)

The IAEA mission urged Japan to set more realistic goals and
adopt decontamination methods that are easier to implement, such
as burying contaminated topsoil underground rather than removing
it altogether. In more direct terms, the IAEA urged Japan to stop
the competition in pandering to media-enhanced irrational fears
of radiation.

Russian Prime Minister, and soon to again be President,
Vladimir Putin, in his first phone conversation with new Japanese
Prime Minister Yoshihiko Noda expressed the hope that the
Japanese Diet would quickly approve a pact on nuclear energy
signed originally between Japan and Russia in 2009. It
establishes a bilateral civil nuclear cooperation agreement with
Russia, thus paving the way for Japan to export a modern nuclear
power plant and other technology to Russia. Prime Minister Noda
has yet to make clear Japan's nuclear export policy.

The bravest are surely those who have the clearest vision of what is before them, glory and danger alike, and yet notwithstanding go out to meet it.- Thucydides

One would think that Russians might have more practical experience with this type of decontamination than many others and this might confer credibility. Any comments?

Most of the clean up, of stuff like top soil, will do far more damage than just letting the levels drop naturally. Levels are so low in most of those areas there's really no reason to even consider it. I'd buy and live in a house in those forest tomorrow.

Wikipedia states about 3.3 mg per cubic meter, other sources say 3 p.p.b. Amount for fuel rod depends upon type reactor selected. One source says 380 lbs of uranium are present in Fukushima fuel rod assembly. As some may know, liquid fuel reactors may be advantageous, allowing xenon generated in fission to bubble out rather than rupturing fuel rods.

Tidal power- tides operate twice per day, so MAXIMUM possible capacity factor is 25% or so. Plus, tidal basins where phenomenon of resonance results in large tides are relatively rare, and extracting energy from resonating systems tends to end resonance altogether.

There are multiple kinds of tidal. One form is just harnessing pressure changes that occur naturally as a wave breaks on the shore. That might operate all day long. Technically it's called "wave power", and wiki puts it in a separate category from tidal.

Wave power - Wikipedia, the free encyclopedia
Another is the tidal barrage. It can kind of work all day long depending on how big the storage is. Just turn the turbines backwards to gradually harness energy all the while the tide is coming in, and then turn them forward to harness the same water as it leaves the storage. You can harness it both ways. The problem is that the height difference is slight if you do that, so not nearly as much energy as you get from damming a river.

What if we build really big floating solar array barges that can travel around the ocean taking advantage of the best weather patterns? They'd have to convert the energy to fuel in order to deliver it to land, of course, but we've already established that that is not an unreasonable constraint.

Some clocks are only right twice a day, but they are still right when they are right.

It is negligible return either way compared to nuclear, plus vulnerable to tsunamis and storms. This exposure can be eliminated for nuclear, as motivated by Fukushima, but necessarily MUST be a factor for wave power or tidal. As for your traveling barges, the same applies to them, plus propulsion requires energy.

Plus the inevitable environmentalist opposition.

In theory river water meeting sea can generate power too, for after all, it requires power to remove salt from solution, but the general objections listed above apply here as well.

Thanks for links and participation, always a pleasure to see you here.

The bravest are surely those who have the clearest vision of what is before them, glory and danger alike, and yet notwithstanding go out to meet it.- Thucydides

It is negligible return either way compared to nuclear, plus vulnerable to tsunamis and storms. This exposure can be eliminated for nuclear, as motivated by Fukushima, but necessarily MUST be a factor for wave power or tidal. As for your traveling barges, the same applies to them, plus propulsion requires energy.

Plus the inevitable environmentalist opposition.

Why would the environmentalists care? We're talking about a barge. A mobile vehicle that presumably leaves nothing in the water.

Some clocks are only right twice a day, but they are still right when they are right.

Both biological and adsorption methods can extract other metals than uranium from the sea, such as cobalt, vanadium, molybdenum, and titanium. Fascinating, and more details will be provided as available.

Last edited by The Finger Prince; October 18th, 2011 at 11:51 AM.

The bravest are surely those who have the clearest vision of what is before them, glory and danger alike, and yet notwithstanding go out to meet it.- Thucydides

Pondering the subject further, it occurs to Prince that uranyl ions should be heavier than water molecules and be in greater concentration at depth, as is the case for phosphate ions.

Since the Kuroshio Current is a warm current it might be worthwhile to bring up cooler waters from the depths in some sort of ocean thermal generating scheme and to direct effluent with ion-rich water toward seaweed beds to improve adsorption and help sell it to the GREENIES, damn them all. Some research has been conducted regarding cultivated seaweed beds as havens for fish, this might help as well.

Unlike wave and tidal generating installations the bulk of the OTEC could be safely anchored in deep water and even submerged to minimize potential damage from storms and waves.

Last edited by The Finger Prince; October 18th, 2011 at 05:11 PM.
Reason: speculation

The bravest are surely those who have the clearest vision of what is before them, glory and danger alike, and yet notwithstanding go out to meet it.- Thucydides